This invention relates to a cathode ray tube (CRT) for color television and allied display applications, employing a shadow mask for color separation, and more particularly relates to such a CRT having an improved shadow mask mounting system.
CRTs for color television, computer monitors and other display applications rely on a cathodoluminescent phosphor screen to provide a visible display. Such a screen is composed of a repetitive pattern of a large number of small red, blue and green-emitting phosphor elements, which are excited to luminescence by electron beams emanating from an electron gun behind the screen. There are three beams, one for each of the red, blue and green components of a color display signal. In operation, the screen is repetitively scanned by the three beams simultaneously, while the intensities of the beams are modulated by the respective individual primary color components of the display signal. The large number of phosphor elements, together with the scanning frequency, results in the perception of a steady, full color display by a viewer.
Such CRTs typically employ a shadow mask to achieve color separation. A shadow mask is a thin sheet having a large number of apertures and mounted between the phosphor screen and the electron gun, a short distance behind the screen. The apertures are aligned with the phosphor elements on the screen and the electron beams are directed from the electron gun to converge at the mask. When the beams pass through the individual apertures, they diverge from one another to land on the phosphor element of the corresponding color.
The mask, which is typically 0.15 to 0.25 mm thick, is supported on a frame to maintain its shape. This frame is then securely mounted in the glass envelope in order to maintain the mask in proper registration with the screen. Such registration must not only be maintained in the X and Y directions, but also in the Z direction, i.e., along the tube axis in order to insure that the beams do not land on adjacent phosphor elements, which would degrade the color purity of the display image.
Particularly during the warm-up period, the mask heats up and expands in all directions. Once the frame also warms up, then the thermal compensation effect of the suspension system takes place, moving the whole mask closer to the screen, maintaining overall color purity by bringing all of the mask apertures back into the electron beam path. When the temperature differential between the mask and frame is large during initial warm-up, the time required for thermal compensation is longer. This differential is minimized by using a frame of as low a mass as possible.
A common technique to maintain the proper Q space (distance between mask and screen) during tube warm-up has been to employ bimetal mounting springs attached to three sides of the frame, which springs are attached to mounting studs embedded in the wall of the glass envelope.
As the mask heats and expands in the X, Y, and Z directions, the mounting springs also heat, and the different expansion rates of the component metals in the springs produce a compensating motion of the entire mask toward the screen.
A more recent design employs a so-called "corner lock" suspension system, in which corner lock mechanisms, each of which include thermal compensation means, are attached to the four corners of the frame. This results in a more stable arrangement than that achievable using the side mounting arrangement, thereby enabling use of a lighter and less costly mask and/or frame.
U.S. Pat. Nos. 3,986,072 and 3,999,098, assigned to Zenith Radio Corporation, disclose corner lock systems wherein the frame is formed integrally with the mask, which is welded to four corner brackets. Cantilevered leaf springs welded to the corner brackets engage legs fixed to the face plate of the vacuum tube adjacent four corners of the display screen. Three of the springs are provided with holes which engage studs on the legs, while a fourth spring is provided with a slot which engages a stud on the fourth leg. This slot permits movement of the mask in the X and Y directions (parallel to the screen) and fix it in the Z direction.
The "frameless mask" of Zenith is formed in a complex shape in order to provide the structural integrity to withstand compressive stresses for mounting without welding. However it is not sufficiently strong to support direct mounting of an internal magnetic shield to the corner brackets.
The current Philips mask suspension system is described in an article by Robert Donofrio entitled "Corner Lock Suspension" in the November 1995 issue of Information Display. This system employs corner brackets welded to lightweight diaphragm strips to form a rectangular frame; each diaphragm strip has an angular cross section formed by a base section and an upright section to which the shadow mask is welded. A resilient plate, also referred to as a temperature compensating plate or as a hinge plate, is fixed to each corner plate by a spring which loads it toward a pin embedded in a corner of the skirt adjacent to the face plate. The pin is engaged by a floating washer mounted to the hinge plate. During assembly, the floating washers are welded to the hinge plates after the mask/frame assembly is engaged to the pins. The phosphor elements are then applied in a photo-lithographic screening process which involves removing and replacing the assembly several times. After a conductive coating is applied to the phosphor elements, the assembly is fixed in place by welding the floating washers to the studs. The internal magnetic shield is fixed in the vacuum envelope independently by separate links which are welded to the studs over the frame assembly.
U.S. Pat. No. 4,652,792 of Toshiba discloses a rectangular frame which is suspended at its corners by spring members which provide geometric temperature compensation during warmup. The frame is 1.6 mm thick and therefore relatively heavy, and generates considerable scrap during manufacture insofar as it is stamped from a single piece and formed without seams.
A corner suspension system of Thompson Consumer Electronics is described in an article by R. C. Bauder and F. R. Ragland entitled "An Improved Shadow-Mask Support System for Large-Size CRTs" in SID Intl. Technical Papers (1990). This system employs bimetal clips welded to the corners of a one-piece frame, and backward extending springs welded to the distal ends of the clips.
Drawbacks of the known systems include difficulty in salvaging masks and frames, where they are welded to the studs; high scrap rates during manufacture; complex parts including bimetallic clips; inability to mount the IMS directly to the frame; or systems with heavy frames, which result in poor color purity from the long warm up times, and instability of the assembly when dropped.